A wide variety of interferometric based imaging techniques have been developed to provide high resolution structural information of samples in a range of applications. Optical Coherence Tomography (OCT) is an interferometric technique that can provide images of samples including tissue structure on the micron scale in situ and in real time (see for example, Huang, D. et al., Science 254, 1178-81, 1991). OCT is based on the principle of low coherence interferometry (LCI) and determines the scattering profile of a sample along the OCT beam by detecting the interference of light reflected from a sample and a reference beam (see for example, Fercher, A. F. et al., Opt. Lett. 13, 186, 1988). Each scattering profile in the depth direction (z) is reconstructed individually into an axial scan, or A-scan. Cross-sectional images (B-scans), and by extension 3D volumes, are built up from many A-scans, with the OCT beam moved to a set of transverse (x and y) locations on the sample.
Many variants of OCT have been developed where different combinations of light sources, scanning configurations, and detection schemes are employed. In time domain OCT (TD-OCT), the pathlength between light returning from the sample and reference light is translated longitudinally in time to recover the depth information in the sample. In frequency-domain or Fourier-domain OCT (FD-OCT), a method based on diffraction tomography (see for example, Wolf, E., Opt. Commun. 1, 153-156, 1969), the broadband interference between reflected sample light and reference light is acquired in the spectral frequency domain and a Fourier transform is used to recover the depth information (see for example, Fercher, A. F. et al., Opt. Commun. 117, 43-48, 1995). The sensitivity advantage of FD-OCT over TD-OCT is well established (see for example, Leitgeb, R. et al., Opt. Express 11, 889, 2003; Choma, M. et al., Opt. Express 11, 2183-9, 2003).
There are two common approaches to FD-OCT. One is spectral domain OCT (SD-OCT) where the interfering light is spectrally dispersed prior to detection and the full depth information can be recovered from a single exposure. The second is swept-source OCT (SS-OCT) where the source is swept over a range of optical frequencies and detected in time, therefore encoding the spectral information in time. In traditional point scanning or flying spot techniques, a single point of light is scanned across the sample. These techniques have found great use in the field of ophthalmology. However, current point scanning systems for use in ophthalmology illuminate the eye with less than 10% of the maximum total power possible for eye illumination spread over a larger area, detect only about 5% of the light exiting the pupil, and use only about 20% of the eye's numerical aperture (NA). It may not be immediately possible to significantly improve these statistics with the current point-scanning architectures since the systems already operate close to their maximum permissible exposure for a stationary beam, suffer from out of focus signal loss, and do not correct for aberrations. Parallel techniques may be able to overcome these challenges.
In parallel techniques, a series of spots (multi-beam), a line of light (line-field), or a two-dimensional field of light (partial-field and full-field) is directed to the sample. The resulting reflected light is combined with reference light and detected. Parallel techniques can be accomplished in TD-OCT, SD-OCT or SS-OCT configurations. Spreading the light on the retina over a larger area will enable higher illumination powers. A semi- or non-confocal parallel detection of a larger portion of the light exiting the pupil will significantly increase the detection efficiency without losing out of focus light. This gain in sensitivity can be traded off for higher acquisition speed. The fast acquisition speed will result in comprehensively sampled volumes which are required for applying computational imaging techniques. Several groups have reported on different parallel FD-OCT configurations (see for example, Hiratsuka, H. et al., Opt. Lett. 23, 1420, 1998; Zuluaga, A. F. et al., Opt. Lett. 24, 519-521, 1999; Grajciar, B. et al., Opt. Express 13, 1131, 2005; Blazkiewicz, P. et al., Appl. Opt. 44, 7722, 2005; Považay, B. et al., Opt. Express 14, 7661, 2006; Nakamura, Y. et al., Opt. Express 15, 7103, 2007; Lee, S.-W. et al., IEEE J. Sel. Topics Quantum Electron. 14, 50-55, 2008; Mujat, M. et al., Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XIII 7168, 71681E, 2009; Bonin, T. et al., Opt. Lett. 35, 3432-4, 2010; Wieser, W. et al., Opt. Express 18, 14685-704, 2010; Potsaid, B. et al., Opt. Express 18, 20029-48, 2010; Klein, T. et al., Biomed. Opt. Express 4, 619-34, 2013; Nankivil, D. et al., Opt. Lett. 39, 3740-3, 2014) and recently a new parallel interferometric Fourier domain imaging technique called partial field holoscopy has been introduced (see for example, PCT Publication No. WO 2015/189174, the contents of which are hereby incorporated by reference).
The related fields of holoscopy, digital interference holography, holographic OCT, and Interferometric Synthetic Aperture Microscopy are also interferometric imaging techniques based on diffraction tomography (see for example, Kim, M. K., Opt. Lett. 24, 1693-1695, 1999; Kim, M.-K., Opt. Express 7, 305, 2000; Yu, L. et al., Opt. Commun. 260, 462-468, 2006; Marks, D. L. et al., J. Opt. Soc. Am. A 24, 1034, 2007; Hillmann, D. et al., Opt. Lett. 36, 2390-2, 2011, and PCT Publication No. WO 2015/189174). All of these techniques fall in the category of computational imaging techniques, meaning that post-processing is typically necessary to make the acquired data comprehendible for humans. They are commonly implemented in full-field configurations, although interferometric synthetic aperture microscopy is often also used in a point-scanning configuration.
As in FD-OCT, partial field systems benefit from the heterodyne amplification by the reference light. Because the reference light is typically much stronger than the sample light, it is also the main contributor of relative intensity noise (RIN). Balanced detection systems have been shown to suppress common mode noise and at the same time improve the dynamic range of the analog to digital conversion due to the subtraction of the DC term. In PCT Publication No. WO 2016/058910, the usefulness of balanced detection systems for partial field holoscopy systems was already discussed. Because of the spatially resolved detectors typically used in such systems it is desirable to use free space optics—rather than fiber optics—balanced detection system. Free space optics implementations of balanced detection systems can however be challenging due to their very strict alignment requirements.